Self driven synchronous rectifier shutdown circuit and method

ABSTRACT

There is disclosed a power conversion circuit comprising a transformer having a primary side driven by an input section and a secondary side connected to first and second self-driven synchronous rectifiers. A shutdown section includes a control section adapted to detect a predetermined condition and means for shorting a winding of the transformer upon detection of the predetermined condition.

RELATED APPLICATION INFORMATION

This application is a continuation of U.S. application Ser. No.10/843,406, filed May 10, 2004, now U.S. Pat. No. 7,203,041 entitled“Primary Side Turn-Off of Self-Driven Synchronous Rectifiers,” which inturn claims priority from U.S. Provisional Application No. 60/567,123filed Apr. 30, 2004 and entitled, “Primary Side Turn-Off of Self-DrivenSynchronous Rectifiers.”

NOTICE OF COPYRIGHTS AND TRADE DRESS

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. This patent document may showand/or describe matter which is or may become trade dress of the owner.The copyright and trade dress owner has no objection to the facsimilereproduction by any one of the patent disclosure as it appears in thePatent and Trademark Office patent files or records, but otherwisereserves all copyright and trade dress rights whatsoever.

BACKGROUND OF THE INVENTION

1. Field

The present invention relates to power converters.

2. Description of the Related Art

Advancements in the electronic arts have resulted in increasedintegration of electronic devices onto reduced circuit form factors.This trend has driven a demand for power supplies that provide very highefficiency. One type of DC/DC power converter—employing self-drivensynchronous rectifiers, provides relatively high efficiency in lowoutput power applications.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a first power conversion circuit.

FIG. 2 is a driving voltage waveform.

FIG. 3 is a graph depicting output and gate drive voltage waveforms of apower conversion circuit without shutdown control.

FIG. 4 is a graph depicting output and gate drive voltage waveforms of apower conversion circuit with shutdown control.

FIG. 5 is a second power conversion circuit.

DETAILED DESCRIPTION OF THE INVENTION

Throughout this description, the embodiments and examples shown shouldbe considered as exemplars, rather than limitations on the apparatus andmethods of the present invention.

Description of Apparatus

Referring now to FIG. 1, there is shown a power conversion circuit 100which may be included in a power converter, such as a DC/DC converter.The power conversion circuit 100 is merely one of many possibleemploying self-driven synchronous rectifiers. The power conversioncircuit 110 may be any transformer isolated topology, including forwardconverters, flyback converters and bridge type converters. The powerconversion circuit 100 may be hard switched, soft switched, and/orresonant. Power conversion circuits may include various solid state,discrete and other components.

The power conversion circuit 100 may include a transformer 110, an inputsection 120 and an output section 130. The input section 100 may be afull bridge primary circuit. The input section 120 may receive power atan input V_(in), and the output section 130 may deliver power at anoutput V_(out). The input power and the output power may be DC, and thepower conversion circuit 100 is adapted to convert the expected inputpower to output power having desired characteristics (e.g., voltage,current).

The transformer 110 is disposed between and coupled to the input section120 and the output section 130. The transformer 110 has a primary side110 a and a secondary side 110 b. The transformer 110 may be ideal,substantially ideal, or not at all ideal.

The primary side 110 a may include a main winding 112 and otherwindings. The secondary side 110 b may include a main winding 113, anauxiliary winding 114 and other windings. The primary main winding 112is included in the input section 120. The secondary side 110 b windings113, 114 are included in the output section 130. The windings 112, 113,114 are shown with the dot convention to indicate polarity. The windings112, 113, 114 may be wound on a common core (not shown). The core may beiron, another magnetic material or otherwise. The transformer 110 mayhave other configurations and materials, and may be replaced with otherdevices providing the same or similar functionality.

The secondary main winding 113 includes an upper terminal 113 u, a lowerterminal 1131 and a center tap 113 c. Thus, the input voltage to thesecondary main winding 113 may be alternated from one to the other to berectified. The upper terminal 113 u and the center tap 113 c define anupper portion of the secondary main winding 113. The lower terminal 1131and the center tap 113 c define a lower portion of the secondary mainwinding 113.

A driving voltage V_(s) is defined across the upper terminal 113 u andthe center tap 113 c. The same driving voltage V_(s) may be presentacross the lower terminal 1131 and the center tap 113 c, though thepolarity of the driving voltage for the lower portion of the mainwinding 113 is opposite that of the upper portion due to theirrespectively opposite polarities. The center tap 113 c may be at otherpositions than the mid-point of the main winding 113, and this and otherreasons may result in differing driving voltages across the upperportion and lower portion of the main winding 113.

The output section 130 may comprise first and second rectifiers 131,132, first, second, third and fourth resistors 133, 135, 137, 138, firstand second capacitors 134, 136 and a choke 139, as well as other circuitelements.

The rectifiers 131, 132 may be MOSFETs having integral reverserectifiers or equivalent switches. The rectifiers 131, 132 haverespective activation terminals, and if embodied as MOSFETS, haverespective gates (corresponding to the activation terminals), sourcesand drains. For convenience, the rectifiers' activation terminals arereferred to herein as gates. A gate drive voltage V_(g) is defined fromthe gate of either rectifier 131, 132 to common. Instead of the MOSFETs,IGBTs, TRIACs, rectifiers in combination with controllable switches(e.g., transistors, relays), and other devices and combinations ofdevices could be used.

The rectifiers 131, 132 may be synchronous and self-driven. They are“synchronous” because, under normal operations, the rectifiers 131, 132are gated on and off to coincide with the operation of MOSFETs in theinput circuit 110 a. They are “self-driven” because their on and offoperation is controlled by the output section 130, rather than by aseparate controller or control circuit (e.g., an external controller).

The upper terminal 113 u is connected to the drain of the secondrectifier 132 and to the first resistor 133. The opposite terminal ofthe first resistor 133 is connected to the first capacitor 134. Thefirst capacitor 134 is connected to the gate of the first rectifier 131,the third resistor 137 and the upper terminal 114 u. The third resistor137 and the source of the first rectifier 131 are both connected tocommon. The capacitors 134, 136 in combination with the resistors 133,135 provide bypasses to reduce noise and provide additional gate drivecurrent to improve efficiency.

The lower terminal 1131 is connected to the drain of the first rectifier131 and to the second resistor 135. The opposite terminal of the secondresistor 135 is connected to the second capacitor 136. The secondcapacitor 136 is connected to the gate of the second rectifier 132, thefourth resistor 138 and the lower terminal 1141. The fourth resistor 138and the source of the second rectifier 132 are both connected to common.

The secondary auxiliary winding includes an upper terminal 114 u and alower terminal 1141. The upper terminal 114 u is connected to the gateof the first rectifier 131. The lower terminal 1141 is connected to thegate of the second rectifier 132.

The center tap 113 c is connected to the choke 139. The output V_(out)is drawn from the choke 139. The choke 139 is a substantially loss-lesspath for direct current and a substantially infinite impedance foralternating current.

The input section 110 may include a power section, which itself may alsobe used as a shutdown section 160. The shutdown section 160 includesupper 161, 162 and lower 163, 164 switches, the primary main winding112, and a control section 165. The switches 161, 162, 163, 164 may beMOSFETs as described above. Although shown integrated with the shutdownsection 160, the control section 165 may be separate from the shutdownsection 160, from the input section 120 and even the power conversioncircuit 100 itself. The control section 165, at appropriate times,causes a short in a primary winding, such as the primary main winding112. This short may be for a full winding, for one or more turns of awinding, or may be effective as a short of a full winding. References to“shorting a winding” and “shorting a turn” are intended to beinterchangeable and to include any type of short which provides thiseffect.

The control section 165 senses for one or more predetermined conditions,and then turns on or turns off the switches 161, 162, 163, 164 as thecase may be. The predetermined conditions may be for example, (a)falling below the undervoltage lockout point (UVLO) level, (b)over-current, (c) over-temperature, or (d) some other designated faultor non-fault condition. The control section 165 may be one or morediscrete and/or integrated devices. The control section 165 may receivepower from the input voltage V_(i), and/or may receive power fromanother source (e.g., a primary auxiliary winding). Instead of sending asignal on an output, the control section may cause activation of theshutdown section (e.g., a turn to be shorted) in a different way.

By turning on the lower switches 163, 164, the primary main winding 112is shorted. This places the transformer 110 into a lower impedance statethat will not allow it to provide the energy needed for the synchronousrectifiers to self-oscillate. Alternatively, the upper switches 161, 162may be turned on and the lower switches 163, 164 turned off to obtainthe same effect.

The shutdown section 160 may include a DC blocking capacitor 150 inseries with the primary main winding 112.

The auxiliary drive winding 114 is not needed to switch the gates of therectifiers 131, 132. They can be driven by the opposite windings or bycoupling the drains to the opposite gates. This may be done, forexample, in low output voltage converters where the voltage per turn isless. Referring now to FIG. 5, there is shown another power conversioncircuit 500 which may be included in a power converter, such as a DC/DCconverter. The power conversion circuit 500 of FIG. 5 is similar to thatof FIG. 1, but lacks the auxiliary secondary winding 114, and attendantRC sections.

The power conversion circuit 500 includes a transformer 510, an inputsection 520 and an output section 530. The input section 520 may receivepower at an input V_(in), and the output section 530 may deliver powerat an output V_(out).

The transformer 510 has a primary side 510 a and a secondary side 510 b.The transformer 510 may be ideal, substantially ideal, or not at allideal. The primary side 510 a may include a main winding 511, anauxiliary winding 512 and other windings. The secondary side 510 b mayinclude a main winding 513 and other windings. The primary side 510 awindings 511, 512 are connected to the input section 520. The secondaryside 510 b winding 513 is connected to the output section 130.

The secondary main winding 513 includes an upper terminal 513 u, a lowerterminal 5131 and a center tap 513 c. The upper terminal 513 u and thecenter tap 513 c define an upper portion of the secondary main winding513. The lower terminal 5131 and the center tap 513 c define a lowerportion of the secondary main winding 513.

The input section 520 includes a shutdown section 525, discussed below.

The output section 530 may comprise first and second rectifiers 531,532, first, second resistors 537, 538 and a choke 539, as well as othercircuit elements. The rectifiers 531, 532 are driven by the mainsecondary winding 513.

Although specific devices are shown in the power conversion circuits100, 800, substitutes and alternatives are possible and may be desirableand necessary.

Description of Processes

The power conversion circuits shown in FIGS. 1 and 5 are similar. Thus,the operation of only the power conversion circuit 100 of FIG. 1 isdescribed, except where there are significant differences. Operation ofthe output section 130 is described with respect to a driving voltagewaveform as shown in FIG. 2. In FIG. 2, the driving voltage V_(s) isdepicted as a series of rectangular pulses having a predetermined dutycycle that alternate between a positive voltage and a negative voltage.The input section 120 may be configured to provide such a waveform.

During the positive portion of the conduction cycle (e.g., time t₁), thedriving voltage V_(s) and the gate drive voltage V_(g) are positive,causing the first rectifier 131 to turn on and the second rectifier 132to turn off. This forms a current path through the lower portion of themain winding 113. Conversely, during the negative portion of theconduction cycle (e.g., time t₃), the driving voltage V_(s) and the gatedrive voltage V_(g) are negative, causing the first rectifier 131 toturn off and the second rectifier 132 to turn on. This forms a currentpath through the upper portion of the main winding 113. Thus, theauxiliary winding 114 provides synchronicity between the first andsecond rectifiers 131, 132 by alternatively connecting the upper andlower terminals 114 u, 1141 of the auxiliary winding 114 to respectivegates of the first and second rectifiers 131, 132. The conduction cyclemay also have dead times (e.g., time t₂, t₄).

After V_(in) is removed (e.g., time t₅), the output voltage V_(out)should also drop to zero. However, stored energy may be present in theoutput section 130, for example in the form of a prebias voltage fromthe output and stored energy in one or more of the capacitors 134, 136.This stored energy can drive the output section 130 such that therectifiers 131, 132 continue to oscillate on/off. The sustainedoscillation at turn off may also occur due to a resonance between thetransformer magnetizing inductance and/or leakage inductance and thegate capacitances of the rectifiers 131, 132.

Thus, the rectifiers 131, 132 form a Royer style oscillator with thetransformer 110, so that V_(out) will be non-zero for a period of timeuntil the stored energy is dissipated to a minimum level (e.g., zero).When the first rectifier 131 is on, energy is pulled through thetransformer 110 and the first rectifier's gate is discharged, turningthe first rectifier 131 off. While this is happening, the secondrectifier's gate is being charged and the second rectifier 132 willturn-on as the first rectifier 131 turns off. The cycle begins anew withthe second rectifier 132 pulling current through the transformer 110,discharging the second rectifier's gate and charging the firstrectifier's gate. This oscillation may be lower or higher frequency thannormal and possibly high power.

The oscillation can be seen in FIG. 3. This graph shows the outputvoltage V_(out) and the gate drive voltage V_(g). (FIG. 3 includes timedesignations t₆, t₇, t₈ and t₉. These time designations are not relatedto the time designations in FIG. 2, or those in FIG. 4.)

Up until time t₆, the output voltage V_(out) and the gate drive voltageV_(g) are roughly constant. At time t₆, power to the input section isremoved and the input voltage V_(g) falls off until time t₇ and rapidlydrops to a minimum at time t₈. When the power converter is turned off,the input voltage V_(in) and output voltage V_(out) will decay to theUVLO. At this point the power converter will shut itself off. At thispoint if the shutdown section 160 is not present the power conversioncircuit 100 may self oscillate as in FIG. 3. It may continue tooscillate until the gate drive voltage V_(g) has decayed to a value thatdoes not permit self-oscillation.

From time t₆ until time t₇, the gate drive voltage V_(g) decreases asthe output voltage V_(out) decreases. At time t₇, the power converterhits the UVLO and the synchronous rectifiers begin to self oscillateuntil a later time t₉ when the stored energy has been discharged to athreshold level.

Once the rectifier oscillation is initiated, the duration for which itcontinues depends on the stored energy in the power conversion circuit,the Q of the gate drive circuit capacitances and the linked inductance.The damping resistance in this circuit is small, and an effective way tostop the oscillations is reduction of the stored energy or the Q of thecircuit.

For purposes of simplicity and clarity, it is enough to consider twocapacitances linked by an inductor. The characteristic impedance Zo ofthe output section 130 is

$\sqrt{\frac{L}{C}}{and}$the Q of the circuit is Zo/R. By “characteristic impedance,” it is meantthe impedance (resistance) to energy transfer associated with wavepropagation in a line that is much longer than the wavelength, therebygiving the ratio of voltage to current when there are no reflections.When the gate drive oscillations set in, the L is nothing but thereflected magnetizing inductance, and C the equivalent gate capacitance.For example, if the reflected L is 4 uH, and effective C is 3000 pF, thecharacteristic impedance is 1333. If the resistance is small, say 1 ohm,the Q is 1333. In short, given the low resistance and high inductance,the stored energy could be large even with a seemingly large gatecapacitance in the circuit.

The situation is more critical in the event that a source is present onthe output, where such oscillations can draw energy from the source andbe self sustaining. In this case, it is imperative that the ratio of thestored energy to the work done is as small as possible, since this couldbe self destructive to the rectifiers 131, 132. Also the power down ofthe power conversion circuit 100 is not desirable since the outputsection 130 continues to switch as long as an external voltage ispresent.

A power conversion circuit, such as that shown in FIG. 1, therefore mayinclude a means for reducing self-oscillation between its firstsynchronous rectifier and second synchronous rectifier after power toits input section has been removed. This may be accomplished in a numberof ways.

A shorted turn may be placed on the transformer. For example the mainwinding or the auxiliary winding of the primary side may be shorted.However, any low impedance winding section can be shorted to yield thesame result, including those of the secondary side. Shorting thetransformer reduces the inductance in the circuit and stops the couplingof energy from one transformer winding to another. Shorting thetransformer and other selected solutions will turn on both synchronousrectifiers leading to a rapid depletion of energy stored in the powerconversion circuit. This solution reduces the characteristic impedanceZo by several orders of magnitude. Thus, energy transfer to the gates ofthe synchronous rectifiers is stopped by placing a shorted turn on theprimary side of the transformer. The shorted turn limits the energytransfer through the transformer to a very low level.

One embodiment of the shorted-transformer solution is the shutdownsection 125 shown in FIG. 1. The shutdown section 125 is connected tothe power input V_(in) so that removal of power may be detected. Theshutdown section 125 is interposed between one or both of the mainwinding 111 and auxiliary winding 112 of the primary side 110 a, andtheir respective connections 121, 122 to the input section 120. When theshutdown section 125 detects that V_(in) has been removed or turned off,the rapid shutdown section 125 shorts one or both of the primarywindings 111, 112.

With this solution, the turn-off oscillation now changes to a higherfrequency, lower power oscillation which involves the synchronousrectifiers operating briefly in the linear region as they turn the poweroff. This allows the voltages on the gates of the synchronous rectifiersto be lowered continuously, and they will fall below the turn-onthreshold of the synchronous rectifiers. With the reduction intransformer/inductor energy the circuit stays in this state for a muchshorter time period. The result is a soft turn-off of the output voltageV_(out).

By shunting the magnetizing inductance and transferring the resonance toa “leakage” inductance, the value of Zo is now

$\sqrt{\frac{L_{lk}}{C}}.$The stored energy is reduced by a factor of

$\sqrt{\frac{\frac{L}{L}}{lk}}.$

FIG. 4 shows that the operation of the synchronous rectifiers quicklystops after the main drive winding is shorted. As in FIG. 3, up untiltime t₁₀, the output voltage V_(out) and the input voltage V_(in) areroughly constant. At time t₁₀, power to the input section is removed andthe input voltage V_(in) falls off to zero at time t₁₁. From time t₁₀until time t₁₁, the gate drive voltage V_(g) decreases as the outputvoltage V_(out) decreases. After the input voltage V_(in) drops to theUVLO at time t₁₁, the output circuit is no longer being supplied powerfrom the primary side of the transformer. At time t₁₁ the shortedprimary winding has caused all oscillations to cease.

In addition to turn-off, a power converter may have other times when acontrolled shutdown may be desired. For example, in some powerconversion circuits, there is a period of dead time during switching.This dead time is shown in FIG. 2 as times t₂ and t₄. Thus, the shutdownsection 160 may be activated (e.g., the control section 165 may send asignal on its output 166) every cycle during the dead time (e.g., timest₂, t₄). This may improve the efficiency of the power conversion circuit100 by increasing its efficiency during switching transitions.

Although exemplary embodiments of the present invention have been shownand described, it will be apparent to those having ordinary skill in theart that a number of changes, modifications, or alterations to theinvention as described herein may be made, none of which depart from thespirit of the present invention. All such changes, modifications andalterations should therefore be seen as within the scope of the presentinvention.

1. A power conversion circuit comprising a transformer having a primaryside and a secondary side an input section connected to the primary sidean output section connected to the secondary side and comprising a firstself-driven synchronous rectifier and a second self-driven synchronousrectifier a shutdown section comprising a control section adapted todetect a predetermined condition means for shorting a winding of thetransformer upon detection of the predetermined condition, whereinshorting the winding on the transformer reduces self-oscillation betweenthe first self-driven synchronous rectifier and the second self-drivensynchronous rectifier.
 2. The power conversion circuit of claim 1wherein the shutdown section comprises means for shorting a main windingof the primary side of the transformer.
 3. The power conversion circuitof claim 2 wherein the input section comprises four switches to drivethe main winding of the primary side of the transformer, said switchesdisposed as an upper pair and a lower pair the shutdown sectioncomprises means for turning on a pair of switches selected from thegroup comprising the upper pair of switches and the lower pair ofswitches.
 4. The power conversion circuit of claim 3 wherein theshutdown section includes a DC blocking capacitor in series with themain winding of the primary side of the transformer.
 5. The powerconversion circuit of claim 1 wherein the shutdown section comprisesmeans for shorting an auxiliary winding of the primary side of thetransformer.
 6. The power conversion circuit of claim 1 wherein thepredetermined condition is selected from the group comprising fallingbelow an undervoltage lockout point level, over-current, andover-temperature.
 7. A power converter comprising the power conversioncircuit of any of claims 1-6.
 8. A method of operating a power converterincluding a transformer comprising a primary side and a secondary side,an input section connected to the primary side, and an output sectionconnected to the secondary side comprising first and second self-drivensynchronous rectifiers, the method comprising sensing a predeterminedcondition shutting down said power converter in response to sensing thepredetermined condition shorting a winding on the transformer inresponse to sensing the predetermined condition, wherein shorting awinding on the transformer reduces self-oscillation between the firstsynchronous rectifier and the second synchronous rectifier aftershutting down said power converter.
 9. The method of operating a powerconverter of claim 8 wherein shorting a winding comprises shorting amain winding of the primary side of the transformer.
 10. The method ofoperating a power converter of claim 9 wherein the input sectioncomprises four switches to drive the main winding of the primary side ofthe transformer, said switches disposed as an upper pair and a lowerpair shorting a winding further comprises turning on a pair of switchesselected from the group comprising the upper pair of switches and thelower pair of switches.
 11. The method of operating a power converter ofclaim 8 wherein shorting a winding comprises shorting an auxiliarywinding of the primary side of the transformer.
 12. The method ofoperating a power converter of claim 8 wherein the predeterminedcondition comprises removal of external power from the input section.13. The method of operating a power converter of claim 8 wherein thepredetermined condition is selected from the group comprising fallingbelow an undervoltage lockout point level, over-current andover-temperature.